Preheat your oven to 400 degrees F. Bring a large pan of salted water to boil. Peel the potatoes and cut into 1/4 inch chunks. When the water has begun boiling, add the potatoes and cook for around 12 minutes or until soft.

Meanwhile in a deep baking dish or ceramic ware dish, stand a box grater in it. Peel the carrot. Grate the celery, carrot, and cheddar on the coarse side of the grater. Use the fine side of the grater to grate the zest from the lemon. Finely grate or chop your chile. Finely chop the parsley leaves and stalks and add these to the pan.

Cut the cod and the salmon into bite-size chunks and add them to the pan with the shrimp. Squeeze the juice of the zested lemon over the pan (make sure you do not add in any seeds). Drizzle with olive oil and add a pinch of salt and pepper. If you are using spinach or tomatoes, add these now. Mix together well.

The potatoes should be cooked by now. Drain the potatoes using a colander and return them to the pan. Drizzle them with olive oil and add a pinch of salt and pepper. Mash the potatoes until smooth and spread evenly over the top of the fish and grated veggies. Place the pan in a preheated oven for about 40 minutes (or until the fish pie is cooked through and is crispy and golden on top).